AbstractLand-atmosphere interactions and feedbacks mirror a dynamically coupled system that functions on timescales from fractions of seconds to millions of years, driven by terrestrial ecosystem processes and modes of climate variability. Land-atmosphere exchanges involve the transfer of mass, energy and momentum and changes to these fluxes can modify regional-scale climate. This thesis aimed to examine the variability and temporal cycles between temperature (Ta), rainfall and vapour-pressure deficit (VPD) as drivers and gross primary productivity (GPP), net ecosystem exchange (Fc), ecosystem respiration (Fre) and latent heat flux (Fe) as ecosystem flux responses, over the 20th century. It also sought to analyse how influential modes of climate variability were to describing variability in ecosystem responses. Climate indices such as the Southern Oscillation Index (SOI), Tasman Sea Index (TSI) and Indonesian Index (II) were chosen to analyse this influence.
The study site is located at Howard Springs, NT (12.5⁰S, 131.2⁰E), chosen for its long-term flux tower and proximity to a Class 1 Bureau of Meteorology station (Darwin). The site is a typical coastal savanna with high annual rainfall (1690mm y-1) experiencing frequent climatic and fire disturbance. Tower flux data trained an Artificial Neural Network (ANN) which was driven by meteorological observations to hind-cast ecosystem responses over a 113 year period. Using a wavelet coherence analysis, interactions between eddy covariance fluxes and meteorological drivers are quantified at multiple time scales over the previous 113 years, using modelled and observational data, in an effort to establish the relationship between climate and the terrestrial carbon and water cycle within tropical north Australia.
Spectral and temporal analysis revealed significant shifts in broad climate from the mid 1960’s onwards with increased temperatures at the Howard Springs site in line with trends observed for Australia, namely a 0.92 oC increase in temperature over the last 113 years. VPD also increased by 24% relative to 1910 and rainfall by 17%. In effect, the Howard Springs site has become hotter and wetter, with an increase in evaporative demand as demonstrated by the increase in VPD. The wavelet analysis suggested increased coupling between the climate and ecosystem exchanges with increased temperature, rainfall and CO2 concentrations. Water fluxes (Fe) showed sensitivity to El Nino phases (drier conditions) while carbon fluxes (GPP, Fre and Fc) consistently correlated with neutral conditions, indicating disconnect between the carbon and water exchanges in response to climate change.
These results can be further developed by analysing additional sites to unpack the mechanisms involved between savanna and climate. The novel approach and unique study period length has contributed to our understandings of the influence of past and present climate variability to savanna land-atmosphere exchanges, providing insight to the behaviour of coastal savanna ecosystems, which covers tens of thousands of square kilometres of tropical north Australia.
|Date of Award||Nov 2014|
|Supervisor||Lindsay B. Hutley (Supervisor)|